Glossary

The email & messaging glossary

Plain-English definitions for the terms behind deliverability, engagement metrics, channels and automation — the vocabulary you need to run messaging that actually reaches people.

Deliverability & authentication

BIMI (Brand Indicators for Message Identification)

BIMI is a standard that displays your verified brand logo next to your emails in supporting inboxes — but only for domains that have enforced DMARC, turning authentication into a visible trust signal.

DKIM (DomainKeys Identified Mail)

DKIM is an email authentication standard that attaches a cryptographic signature to every message, letting the receiving server verify the mail really came from your domain and wasn't altered in transit.

DMARC

DMARC is a policy layer on top of SPF and DKIM that tells receivers what to do with mail that fails authentication — monitor, quarantine or reject — and sends you reports on who is sending in your domain's name.

Double opt-in

Double opt-in is a subscription method where a new contact must confirm their address by clicking a link in a verification email before they're added to your list — proving the address is real and truly consented.

Email blocklist (blacklist)

An email blocklist is a database of IP addresses or domains flagged for sending spam; mailbox providers check these lists and may reject or spam-filter mail from any sender that appears on one.

Email deliverability

Email deliverability is the ability of your messages to reach the recipient's inbox rather than the spam folder or being blocked entirely — measured by inbox placement, not just whether the message was accepted by the server.

IP warm-up

IP warm-up is the practice of gradually increasing send volume from a new sending IP address over days or weeks, so mailbox providers learn to trust it instead of flagging a sudden spike as spam.

Sender reputation

Sender reputation is the trust score mailbox providers assign to your sending domain and IP based on how recipients react to your mail — high reputation means the inbox, low reputation means the spam folder.

Spam trap

A spam trap is an email address created or repurposed by mailbox providers and anti-spam organisations solely to catch senders with poor list hygiene — mailing one signals you don't have genuine permission, and can badly damage your reputation.

SPF (Sender Policy Framework)

SPF is an email authentication standard that lists, in your DNS, exactly which mail servers are allowed to send messages for your domain — so receivers can reject mail from any server not on that list.

Metrics & engagement

Bounce rate

Bounce rate is the percentage of sent emails that couldn't be delivered and were returned by the receiving server, split into hard bounces (permanent failures) and soft bounces (temporary ones).

Click-through rate (CTR)

Click-through rate is the percentage of delivered emails in which the recipient clicked at least one link — the clearest signal of genuine interest, since a click requires deliberate action.

Click-to-open rate (CTOR)

Click-to-open rate is the percentage of recipients who clicked a link out of those who opened the email — isolating how compelling the content and call-to-action were, independent of the subject line or deliverability.

Conversion rate

Email conversion rate is the percentage of recipients who completed the action a campaign was designed to drive — a purchase, signup or download — making it the metric most directly tied to revenue.

Open rate

Open rate is the percentage of delivered emails that recipients opened, calculated as unique opens divided by delivered messages — a headline gauge of subject-line appeal and sender trust.

Unsubscribe rate

Unsubscribe rate is the percentage of recipients who opted out after a send — a health signal for list relevance and sending frequency, and a far safer way to lose contacts than having them mark mail as spam.

Channels & delivery

Email API

An email API is a programmatic interface that lets your application send email (and read delivery events) with a few lines of code — the modern alternative to raw SMTP for transactional and high-volume sending.

SMTP (Simple Mail Transfer Protocol)

SMTP is the internet standard protocol for sending email between servers — the transport layer that carries a message from your application or client to the recipient's mail server.

Transactional email

Transactional email is a message triggered by a user's action or account event — order confirmations, password resets, receipts, shipping updates — sent one-to-one in response to something the recipient did.

Marketing & automation

A/B testing (split testing)

A/B testing is comparing two versions of an email — differing in one variable like the subject line, content or send time — by sending each to a sample and measuring which performs better before rolling the winner out.

Drip campaign

A drip campaign is a series of pre-written messages sent automatically on a schedule or in response to triggers — 'dripping' relevant content to contacts over time instead of one manual blast.

Email personalization

Email personalization is tailoring a message to the individual recipient using their data and behaviour — name, past purchases, location, interests — so the content feels relevant rather than mass-produced.

List segmentation

List segmentation is dividing your audience into targeted groups based on shared traits or behaviour — demographics, purchase history, engagement, lifecycle stage — so each message is relevant to who receives it.

Marketing automation

Marketing automation is software that sends messages and runs marketing workflows automatically based on triggers, rules and customer behaviour — replacing manual, one-off sends with logic that runs itself.

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Email Marketing Glossary — Deliverability, Metrics & Terms — Climails